I've found a discussion on Nishimura's site (vintagepens) about arthritis and Jumbo pens. It seems it's only a rumor ... (Giovanni - you took part in that discussion back in 2016 :-))
Here's another Japanese novelty Fountain pen, a miniature Japanese doll pen (under 3" long capped).
It seems these were meant to be sold to tourist and there are a vast variety of them. I thought it to be an eyedropper, but it is a bulb filler (complete with breather tube.
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www.newpentrace.net